Spectral sensitization of silver halides with sensitizing dyes is a well-known technique. Generally employed dyes for spectral sensitization include methine dyes, such as cyanine, merocyanine, complex cyanine, and complex merocyanine dyes, etc. These dyes may be used in combinations thereof for the purpose of expansion of a color sensitive wavelength region or supersensitization.
Any of these sensitizing dyes is required to have adsorbability onto silver halide grain surfaces to function as a dye of electron injection type. On the other hand, however, it is known that the sensitizing dyes have their limit in adsorption to silver halide grain surfaces, and adsorption to saturation or near saturation often results in serious desensitization (inherent desensitization), as described, e.g., in W. C. Lewis et al., Photographic Science and Engineering, Vol. 13, p. 54 (1969). Moreover, surface coating of silver halide grains with the sensitizing dyes is sometimes accompanied by problems, such as development inhibition. Under the present invention, therefore, the individual silver halide grains exhibit an extremely low rate of absorption (utilizing efficiency) of incident photons in the spectral sensitization region.
Bird et al. proposed to increase the quantity of absorbed photons by having plural dyes adsorbed on silver halide to form multiple layers as disclosed in U.S. Pat. No. 3,622,316 or by having sensitizing dye molecules containing plural cyanine chromophoric groups adsorbed on silver halide as disclosed in U.S. Pat. No. 3,622,317, to thereby effect sensitization utilizing Forster type excited energy transfer. However, these techniques still undergo the aforesaid limitation of adsorption surface area and the disadvantages due to inherent desensitization and attain virtually no substantial positive effects.
Steiger et al. proposed a sensitization technique in which a fluorescent dye, such as a cyanine dye, a xanthene dye, etc., is chemically bonded to colloidal molecules of a dispersion medium, such as gelatin, and the dye bound to, e.g., gelatin, excites the dye adsorbed on the silver halide surfaces or a spectral sensitizing dye of a different kind through Forster type energy transfer [cf. Th. Forster, Disc. Faraday Soc., Vol. 27, 7 (1959)] or optical absorption of luminescence emitted from the dye bound to gelatin as disclosed in Photo. Sci. Eng., Vol. 27, 59 (1983) and Japanese Patent Application (OPI) No. 117619/76 (the term "PI" as used herein means "unexamined published Japanese patent application"). This technique differs from the system of Bird et al. in that a dye which is not directly adsorbed on silver halide grains also contributes to sensitization. However, since the sensitizing dye to be dispersed in a medium according to the method of Steiger et al. naturally exhibits strong adsorbability, a part of the dye bonded to gelatin is also adsorbed directly on the silver halide grains and thereby acts as an energy acceptor. As a result, it is generally difficult to realize ideal overlapping of the luminescence band by the non-adsorbed dye and the absorption band by the adsorbed dye.
This difficulty greatly bars highly efficient energy transmission, because energy transmission essentially requires, in principal, an overlap of a luminescence band and an absorption band, whether it is effected by Forster type energy transfer or reabsorption of luminescence. Besides, when the dye to be used is of such a type that adsorption onto the silver halide grains brings about desensitization, the above-described method cannot be applied. Further, this method involves complicated steps, such as synthesis or purification of a dye capable of being bonded to a dispersion medium, entailing greatly increased production cost.
Moreover, in carrying out this method, it is necessary to use a light-harvesting dye in high concentration. However, the rate of reaction between the dye and dispersion medium molecules has its limit. Even if a high rate of reaction may be achieved, functional groups for hardening wuuld be lost upon reacting, so that a sufficient degree of hardening is hardly obtained. Because of these restrictions on their practical use, there is a limit in concentration of the dye to be added.
Further, the freedom for synthesis and selection of the aforesaid luminescent dye materials capable of being bonded to a dispersion medium is far narrower in scope than that permitted in the technique of the present invention in which an optional amount of a water-soluble luminescent dye is merely added and dispersed in a hydrophilic medium.
The luminescent dye is required to almost completely decolorize during photographic processing. However, that is impossible or needs a special processing step when the dye is chemically bound to the medium.
The above-described sensitization method of utilizing dye adsorption in multiple layers and the method of using a dye bound to a binder both lack the ability to increase sensitization efficiency by separating the function of a spectral sensitizing dye (electron injection type) in an adsorbed state from the function of a light-harvesting dye of the energy transmission type on their functions. These methods are also disadvantageous in that a complicated synthesis of the dye is involved or general development processing is inapplicable.